An acoustic wave device using a piezoelectric thin-film resonator is used as a filter provided in wireless communication equipment. The piezoelectric thin-film resonator includes a film buck acoustic resonator (FBAR) and is configured to have a piezoelectric thin-film having opposite surfaces on which upper and lower electrodes are respectively provided. The resonance frequency of the piezoelectric thin-film resonator depends on the thickness of a portion in which the upper and lower electrodes face each other through the piezoelectric thin-film. Such a portion is referred to as a resonance portion. In a case where the upper and lower electrodes are respectively made of different materials, the resonance frequency of the resonance portion depends on the materials as well as the film thickness.
Japanese Patent Application Publication No. 2002-335141 (Document 1) discloses an art in which a mass loading film is formed in the resonance portion in order to change the resonance frequency of the piezoelectric thin-film resonator. Documents 2 through 5 described below disclose an art of forming an opening in the mass loading film in the resonance portion (Document 2: Japanese Patent Application Publication No. 2002-515667, Document 3: U.S. Pat. No. 6657363, Document 4: Japanese Patent Application Publication No. 2008-172494, and Document 5: International Publication No. 2010/061479). Japanese Patent Application Publication No. 2007-535279 (Document 6) discloses an art of forming a stripe-shaped mass loading film in the resonance portion. According to Documents 1 through 6, the resonance frequency is adjusted for each resonator.
According to the art disclosed in Document 1, resonators having different resonance frequencies are realized by forming mass loading films having different thicknesses. Such mass loading films are formed by performing a series of a film growing process, a photolithography process and an etching process multiple times. This complicates the manufacturing process and increases the production cost. According to Documents 2 through 6, the presence of the mass loading film in the resonance portion formed by patterning increases the spurious intensity, which leads to a deterioration of the resonance characteristic. As a result, the frequency adjustable range becomes narrow.